|World War II aircraft|
|Country of origin||United States|
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The P-39D in World War II
In 1937, the United States Army Air Corps issued a specification for a new fighter via Circular Proposal X-608. It was a request for a high-altitude interceptor aircraft having "the tactical mission of interception and attack of hostile aircraft at high altitude". Specifications called for a maximum airspeed of at least 360 miles per hour (580 km/h) at altitude, and a climb to 20,000 ft (6100m) within 6 minutes; the toughest set of specifications USAAC had presented to that date. Other competing designs included the Curtiss P-40, an outgrowth of a previous design, and the Lockheed P-38, which utilized a complex twin-engine twin boom configuration. Although Bell's limited fighter design work had previously resulted in the unusual Bell YFM-1 Airacuda, the Model 12 proposal adopted an equally original configuration with an Allison V-12 engine mounted in the middle of the fuselage, just behind the cockpit, and a propeller driven by a shaft passing beneath the pilot's feet under the cockpit floor.
The main purpose of this configuration was to free up space for the heavy main armament, a 37 mm Oldsmobile T9 cannon firing through the center of the propeller hub for optimum accuracy and stability when firing. In fact, the entire design was made to accommodate this gun in the aircraft. This happened because H.M. Poyer, designer for project leader Robert Woods, was impressed by the power of this weapon and he pressed for its incorporation though the original concept had been a 20-25 mm cannon mounted in a conventional manner in the nose. This was unusual, because fighters had previously been designed around an engine, not a weapon system. Although devastating when it worked, the T9 had very limited ammunition, a low rate of fire, and was prone to jamming.
A secondary benefit of the mid-engine arrangement was to create a smooth and streamlined nose profile. The weight distribution necessitated a tricycle undercarriage, a first among American fighters, concurrent with the Lockheed XP-38. Entry to the cockpit was through side doors (mounted on both sides of the cockpit) rather than a sliding canopy. Its unusual engine location and the long driveshaft caused some pilot concern at first, but experience showed this was no more of a hazard in a crash landing than with an engine located forward of the cockpit. There were no problems with propshaft failure.
As originally designed, the XP-39 had a turbocharger with a scoop on the left side of the fuselage; both were deleted for production. The production P-39 retained a single-stage, single-speed supercharger with a critical altitude (above which performance declined) of about 12,000 ft.
The XP-39 made her maiden flight on 6 April 1938 at Wright Field, Ohio, achieving 390 mph at 20,000 ft. (630 km/h at 6,100 m), reaching this altitude in only five minutes. The Army ordered twelve YP-39s (with only a single-stage, single-speed supercharger) for service evaluation and one YP-39A. After these trials were complete, which resulted in detail changes including deletion of the external radiator, and on advice from NACA, the prototype was modified as the XP-39B; after demonstrating a performance improvement, the 13 YP-39s were completed to this standard, adding two .30 cal. (7.62 mm) MG to the two existing .50 cals. Lacking armor or self-sealing fuel tanks, the prototype was 900 kg lighter than the production fighters.
- Historical documents